Mobile data and voice communication is significantly growing. With increasing popularity of data and voice communication, it is more likely that the communication needs of a large number of users must be met which are all located within a small area, a place referred to as dense crowd scenario in the art. Typical examples include sport arenas or large office buildings.
In order to increase data transmission performance and reliability, the so-called multiple-input and multiple-output (MIMO) technology may be used in wireless radio telecommunication for transmitting information between a base station and terminals of users. The MIMO technology is therefore known in the industry as one of the potential candidates for evolving mobile communication systems. MIMO systems may use multiple send and receive antennas for wireless communication at a base station. The MIMO technology forms the basis for coding techniques which use the temporal as well as the spatial dimension for transmitting information. The enhanced coding provided in MIMO systems allows a quality and data rate of the wireless communication to be increased.
In a so-called massive MIMO system, the base station may include a large number of antennas, for example several tens or even in excess of one hundred antennas with associated transceiver circuitry. The large number of antennas of the MIMO base station allows radio energy to be spatially focused with improved capacity and radiated energy efficiency. In order to adapt the transmit signal at each individual antenna of the base station in accordance with the currently active receiving terminals, a base station logic needs information about radio channel properties between the terminals and the antennas. A pilot signaling scheme may be used for this purpose which allows the base station to adjust configuration antenna parameters for transmitting signals so as to focus radio energy at terminals or for receiving radio signals. In a MIMO system, pilot signals may be transmitted from all terminals within the cell and possibly also neighboring cells in a time slot which is dedicated to the respective terminal. The pilot signals need to be orthogonal in order for the base station to identify the configuration parameters of the plurality of antennas for each of the terminals. Orthogonality may be achieved for example by using time division multiple access (TDMA), code divisional multiple access (CDMA) or frequency division multiple access (FDMA) technologies or a combination thereof.
In case the MIMO system uses for example a time division duplex (TDD) technology for communicating data between the base station and the terminal, each terminal may transmit a pilot signal in a designated time slot, which can be received by the antennas and analyzed by the base station logic. It will be appreciated that time slots are only one example for orthogonal channels, which orthogonality being attained in the time domain. In order to not interfere with each other, a certain time period can be assigned in each system frame where each terminal may transmit its pilot signal. The time slots in which terminals may transmit their pilot signals in combination are also referred to as a pilot portion of the frame and may be a part of a header of the frame. The remaining time slots of the frame may be used for downlink (DL) and uplink (UL) data transmission. The DL and UL transmissions may be performed in a plurality of time slots which may follow the header of the frame. The pilot signals may each include a training sequence, which the pilot signal received at the plurality of antennas of the base station being analyzed by the base station logic. The base station may use the results of the analysis to determine configuration parameters for transmitting signals via the antennas to the respective terminals or for receiving signals via the antennas from the respective terminals.
Massive MIMO systems may be deployed in buildings such as office buildings, shopping malls and so on, in which a large density of users can occur. In such environments a large number of terminals may be located in a single cell served by the MIMO base station. The time required for the pilot signaling of the terminals in each frame may increase with the number of terminals. For a large number of terminals, the time required for all terminals to transmit their pilot signals may exceed the available pilot signaling time in each frame. While the pilot signaling time, i.e. the number of time slots allocated to pilot signaling, may be adjusted dynamically, the transmission of payload data may be negatively affected if the pilot signaling time was increased too much. Therefore, several terminals may be grouped or clustered to share one of several orthogonal channels for pilot signal transmission. For illustration, in TDD, several terminals may share the same pilot time slot. The several terminals may be assigned to the one of the several orthogonal channels for pilot signal transmission. For example, the several terminals may be assigned to the same pilot time slot in such a manner that the several terminals do no longer transmit their respective pilot signals in each frame, but alternating transmit their pilot signal in the assigned time slot. Similar assignments can be made for other types of orthogonal channels. The several terminals may transmit their pilot signals in a round-robin fashion. Thus, resources for pilot signal transmission, for example orthogonal channels, may be saved or used more efficiently. Thus, grouping or clustering of terminals is one option to handle limitation in the spatial resolution.
However, further obstacles may occur in MIMO systems in view of resources to be used for transmitting pilot signals, when a plurality of base stations of neighboring cells is considered. The resources, for example time slots or otherwise orthogonal channels, may be distributed over the base stations to avoid that a pilot signal from one terminal to a first base station is contaminated by another pilot signal from another terminal sent to a second base station using the same resource. However, this may limit the available resources for transmitting pilot signals.
Therefore, there is a need in the art for methods and devices which address at least some of the above shortcomings in MIMO systems. There is in particular a need in the art for allocating resources for pilot signaling to terminals of a multiple-input and multiple-output (MIMO) system in which a plurality of base stations are serving a plurality of neighboring cells and the total number of terminals becomes large, for example larger than the number of available time slots in a pilot portion of a frame.